A corresponding cup is for example disclosed in EP 1 785 265 A1. This application also discloses a device for producing a stacking projection on a container wall. A corresponding device or tool comprises a mandrel and a support ring which is open at its top. The two parts are movable relative to one another between a stand-by position and a deformation position. The mandrel comprises a retaining indentation running externally circumferentially and the support ring comprises at least in some places a notch projection running internally circumferentially. By the interaction of the mandrel and the support ring in deformation position the stacking projection is produced.
Another stacking projection is for example disclosed in DE 10 2004 056 932 A1 or also in FR 1 181 342. However, corresponding references disclose a holding of a cup bottom between two tools and a radial pressing of a part of the cup wall in direction to the inner of the cup to form a corresponding stacking projection or ledge which is used as a supporting shoulder for a bottom edge or the like of second cup inserted in the first cup for stacking of the cups.
Any radial compression of a part of the wall of the cup will result in a stretching of the material. Such stretching may cause an inner layer of polymer applied to the paper material of the cup to thin out and sometimes make pinholes which may cause leaking. Moreover, any paper that is stretched looses some of its mechanical strength and becomes weaker.